1. Field of the Invention
The present invention generally relates to a control circuit for a switch unit of a clocked power supply circuit as well as to a resonance converter, specifically to a regulated resonance converter.
2. Description of Prior Art
In a multitude of applications, it is required to generate an output voltage or an output current because of an input-side energy source, it typically being required to regulate the output voltage, the output current or the output power.
Solutions which are current today in terms of operating regulated power supply units are flyback converters with galvanic separation, or galvanic isolation. What is disadvantageous is the high level of power dissipation of conventional transformers which arises here, as well as the structural height of conventional electromagnetic transformers, which is several millimeters and is a factor of interference for small powers of up to about 100 Watts, but in particular up to 10 Watts. These disadvantages may be remedied by employing higher-cost planar transformers or integrated magnetic devices (so-called integrated magnetics), by means of which the structural height of such power supplies may indeed be reduced, but the efficiency factor can be influenced to a lesser extent, however, in particular with very small powers of up to 10 Watts. Common flyback converter solutions, however, are advantageous in that output voltage regulation of a precision of about 10% may be achieved by a so-called primary current regulation in that only the switch current (primary current) is observed and used for regulating the output voltage and/or the output current. Please see DE 10143016A1 for details.
A remedy having the goal of reducing the structural height and improving the efficiency factor may be provided by a resonance converter using a piezo transformer. With appropriate dimensioning, said resonance converter has a high efficiency factor of 97% to 99% and can be limited to a structural height of 1 to 5 mm in the entire power range of up to 100 Watts, whereas conventional transformers have structural heights of between 7 mm and 15 mm in this power range. Integrated magnetics may achieve smaller structural heights, for example between about 3 mm and 10 mm, but the efficiency factor of the magnetic transformers mostly does not exceed 90% for powers below 10 Watts. In addition, the technological basic expense for constructing integrated magnetics in the power range below about 50 Watts is clearly too high in comparison with a discretely structured circuit.
The mechanical dimensions of the footprint of the piezotrafo (PT) are dependent on the frequency and may be reduced to values of between 10 mm and 40 mm in the power range mentioned if an expedient frequency range is selected for such applications (e.g. 25 to 500 kHz).
If a half-bride topology or a push-pull topology is used for such a resonance converter, regulation of the output voltage mostly is possible only by feeding back the voltage via a galvanically separating optocoupler or by another galvanically separating device when a primary-action resonant circuit having oscillations is used between an input-side magnetic choke coil and the input capacitance of the piezo transformer (PT).
One has known of various solutions wherein phase shifts between a load quantity (load current) and a voltage quantity of the switch unit in resonance converters are utilized for regulating the output voltage, the output power or the output current.
U.S. Pat. No. 6,002,214 proposes to detect the voltage present across a switch unit, for example in a resonant half-bridge or bridge converter, and to compare the phase position of turning on or off, or switching on or off, this voltage with the phase position of the load alternating current. A phase difference is determined from a comparison of a zero crossing of the load current, which is detected by a sense resistor either in the switch unit or in the load circuit, and the switching signal of the switch voltage (to close or to open the switch). The phase difference is compared with a target phase and fed back, via a regulator, to a voltage-controlled oscillator (VCO) which controls the switch unit. Such a load circuit may operate with low or high Q, so that the phase difference reflects the power supplied to the load.
Mostly, however, regulation is used with low Q of the load circuit, so that the load circuit does not represent a sinusoidal current source in every case, but may also represent a different periodic alternating source. However, a disadvantage of this configuration is that even though the power present at a load fed with alternating current may be regulated via this phase shaft and/or phase difference, the load-circuit elements L and C must be known with a relatively high level of precision for a certain power to be set. If, however, the value of L is not known, the power cannot be adjusted in an exact manner.
In accordance with DE 696 04 896 T2, the phase position of the output voltage of a piezo transformer (piezoelectric converter) is compared to the phase position of its input voltage so as to set (adjust, or regulate) a predetermined value of the phase position that will guarantee optimum efficiency of the converter (luminosity of a cold cathode tube) and, at the same time, constant output power (luminosity) of the converter, which is maintained irrespective of input voltage fluctuations. However, the input voltage range is limited, and the transformation ratio, or transmission ratio, of the converter causes an upward transformation. Therefore, what needs to be detected in addition to the phase signal between input voltage and output voltage is the output current so as to maintain the output power of the load connected (cold cathode tube) at a constant level. Thus, two feedback circuits are required which also do not achieve galvanic separation between input and output.
In accordance with U.S. Pat. No. 6,013,969, a load alternating current is again detected, and the phase position thereof is compared with the voltage present at the switch unit so as to operate, via a regulator (integrator), a voltage-controlled oscillator (VCO) for controlling the switch unit (drive circuit) in a closed loop. Use is made of an input-side boost converter which causes signal matching toward the input, so that the output power is maintained at a constant level and/or so that different loads can be operated within a wide range of load resistances and input voltages. However, in addition to a phase detector, which compares the phase between a switch voltage and a load current, a rectifying circuit is also used, which resistively loads the tapping of the load (sense resistor), and which would thus corrupt the signal of the load circuit if one wanted to couple out, or extract, or tap, this phase signal from the piezo transformer itself. Therefore, one cannot achieve galvanic separation between the load and the input without using an additional optocoupler or another galvanically separating coupling circuit, which is also not required in the application set forth in the document mentioned.
Similarly, in accordance with U.S. Pat. No. 6,348,755 B1, a phase comparison between an input voltage curve of a PT and the input current curve (possibly using a load compensation circuit for correcting the phase curve of the input current of the PT by means of the load current at the output of the PT in the load) forms the signal for driving a voltage-controlled oscillator (VCO) via a low-pass filter. The output load current is detected, in addition, to generate a burst-mode pulse width modulation (burst-mode PWM), which maintains this current at a constant level. Regulating the oscillator frequency via the phase comparison servers to drive the PT in an optimum manner and at a high level of efficiency. The phase position between the input voltage and the input current of the PT is always regulated to a maximum. The pulse width modulation (PWM) has a lower frequency, and it either connects the VCO through to the output, or it switches it off so as to thus maintain the output current at a constant level on average.
In a further solution in accordance with U.S. Pat. No. 6,144,139, a phase difference between a target signal (e.g. a signal of a capacitive current as a phase-related input voltage mapping) and the current present at the input of the PT is used for driving a VCO, wherein subsequently, the duty cycle of the signal is generated via the evaluation of the output voltage by means of an error comparator. Thus, the VCO is generally driven by a phase difference between the input voltage and the input current or between the input voltage and the output current. The output voltage or the output current, on the other hand, are used to set the associated duty cycle of the driver circuit. However, by doing this, a current from the transformation network of the PT is not included in the regulation, so that a galvanic separation of this circuit without galvanically separating feedback elements is not possible, since a output quantity, or variable, is always required for regulation (frequency and duty cycle). Galvanic separation is not envisaged in the solution mentioned. As regards the galvanic separation, the same applies as with the solution in accordance with U.S. Pat. No. 6,348,755 B1.
A similar solution is shown in EP 0 782 374, wherein a phase difference between the input voltage and the output voltage serves to control a VCO, and wherein the output current sets the duty cycle via a pulse width generation. This circuit is also not suitable for galvanic separation without feeding-back elements from the output to the input.
In another solution as is shown in U.S. Pat. No. 6,239,558 B1, an alternating output current present at the load, or a current flowing through the load is detected, and the detected signal is rectified so as to operate a regulator via the comparison with a reference signal.
Another configuration of the prior art has used the detection of the load current of the load network, and has compared its phase position with the switching signal present at the switch unit, for example with the phase position of the turn-off moment, or turn-off instant (Yan Yin; Zane: “Digital Controller design for electronic ballasts with phase control”, PESC 2004, Vol. 3, pp 1855ff, 20-25 Jun. 2004, Aachen, proceedings).
In addition, there are solutions for detecting the voltage present at the load, and/or of the current flowing through the load, wherein a phase difference as compared with the voltage signal present at the converter input is formed from one of these signals so as to control or to regulate the power, the voltage or the current present at the load. In accordance with U.S. Pat. No. 5,866,968, a signal which is proportional to the alternating output voltage is fed back and is compared to the phase of the switching signal of a driving VCO. Optionally, a rectified signal which is proportional to the output voltage is detected and is used for regulating the output voltage or power in addition to the fed-back phase signal.
All of these configurations have in common that only a current or a voltage of the load network is compared with a voltage quantity or a turn-on and/or turn-off quantity of the switch unit, which may be observed at the switch unit.
Thus, previous solutions have made use of a phase shift between a quantity, or variable, of the switch unit and a magnitude of the load current in the load network as a basis of regulating the load. This configuration, however, entails major disadvantages. On the one hand, in a load resonant circuit of low Q in the normal load operation, or nominal load operation, a distortion of the ohmic load current by a dynamic non-linear load (for example CCFL, FL, HID lamps) as compared with the parallel capacitive current (for example a heating circuit capacitor) renders such a regulation via such a phase shift too imprecise, so that another solution in accordance with U.S. Pat. No. 6,002,214 has often been selected instead. With light applications (fluorescent lamps), galvanic separation between the load and the source is not required in most cases, so that a feedback need not be effected via an insulating transformer. In addition, U.S. Pat. No. 6,002,214 would also be applicable with a galvanically separating transformer arranged toward the load, since what is dealt with is an alternating current load, and since a symmetrical half-bridge circuit forms the switch unit. For direct current loads connected downstream from a rectifier bridge with a buffer capacitor, this circuit might react no longer to dynamic changes to the load in such a manner that it is known, by means of the phase position determined, whether only a dynamic load current for loading the buffer capacitance flows via the rectifier, or whether the load has increased permanently. In this case, the output voltage is not so easy to regulate in a dynamic manner.
In addition, the submitted contribution for the IEEE Transactions on Power Electronics of Sep. 7, 2004 entitled “Digital Controller Design for Electronic Ballast with Phase Control” describes a transformational detection of the load current (i.e. a detection performed by a transformer). It is suggested to compare the zero crossing of this load current with the zero crossing of only the active current in the load with regard to their phase shift, and thus to set a constant level of power at the load by means of a constant phase difference thus detected. However, what is disadvantageous is that it is necessary to generate the load current from an additional device which acts as a transformer and is not already included in the load circuit.
In accordance with U.S. Pat. No. 5,866,968, in addition, a signal directly detected from the output voltage is rectified, if need be, which gives rise to a power loading at the output. Said power loading is acceptable only with upward-transforming applications so as to keep respective losses within certain limits. With downward-transforming applications, however, a loading at the auxiliary output by a resistive load or a rectification is problematic, since one would have to keep the voltage at a correspondingly low level to realize low-loss tapping. Consequently, however, the signal-to-noise ratio is too small to be able to evaluate the auxiliary signal in a reliable manner. In addition, however, the phase signal which has been coupled out from an auxiliary tapping of a piezo transformer is compared only with a phase signal of the driver circuit (turn-on moment or turn-off moment) so as to achieve a phase regulation. For this purpose, in the U.S. Pat. No. 5,866,968, an adapted phase rotation across, for example, an RC network is used so as to couple the frequency of the oscillator of the driver circuit of a piezo converter to the frequency of the piezo transformer by means of a phase linking. At the same time, however, the turn-on time is fixedly set (to about 40%) so as to achieve a zero-voltage switching (ZVS) across a sufficiently large load range. If, however, the input voltage is also changed on a large scale, the publication mentioned offers no satisfying solution. Even though a PLL function is used, the turn-on point is suboptimal in the solution presented when a load change in the broad range is added to by a major change of the input voltage. In addition, it is not possible to detect, via the phase signal of the output or of the auxiliary output which has thus been generated, whether zero-voltage switching (ZVS) is still possible, or whether, for example with a small load and a high input voltage, the relative turn-on time approaches zero, so that continuous operation is no longer possible, and so that one would have to switch to burst mode.
It is also desirable to use the phase signal of an auxiliary tapping—the phase signal being proportional to the output voltage or having a fixed mathematical relationship therewith—not only in a PLL loop for an alternating current load so as to set an approximately constant output current or a constant power, but to obtain, also at direct current loads operated by a rectifier at the output of a piezo transformer, a statement, which is independent of the input voltage, about the magnitude of the load by evaluating a suitable phase angle.
If, as is shown in U.S. Pat. No. 5,866,968, however, the moment of switching the switch unit of the converter is compared with the phase position of the output signal (for example of the output voltage), one will obtain a transformation behavior which is dependent on the input voltage and is set to a maximally transferable power and to an optimum efficiency factor of the piezo transformer. In many cases, however, the transformation behavior desired, for example in the over-resonant frequency range, is one which indeed signifies a slightly smaller efficiency factor, but does not entail any increased losses and thus enables the output voltage to be regulated by means of a frequency change without requiring, with smaller loads than the nominal load of the piezo transformer, a burst mode control, which signifies additional oscillation-buildup losses and increased buffer capacitance at the output.
In addition, one method of the load detection which is independent of the input voltage is applicable only if one can unambiguously detect the load current in the switch unit, with regard to its phase position, in its relation to the phase position of the resistive and reactive portions, respectively, of the currents and/or voltages present at the load, so that currents which are superimposed at the input side or at the load network side do not corrupt the zero crossing of the load current which is detected, for example, in the switch unit of a converter, or so that it becomes possible to unambiguously determine and correct this corruption.
In a half-bridge circuit, an input-side corruption of the load current detection in terms of its phase position is impossible whenever at least one of the switches exhibits a parallel capacitance which is shorted in the turn-on interval, and whenever the load current—in the event that such a switch is turned on—flows exclusively through this switch rather than through further parallel or serial reactive elements which are inserted in the load circuit and which otherwise would dissipate some of the load current in parallel with the switch. However, should such reactive elements be present, a correction of the phase position of the load current detected in at least one switch is necessary, so that one may infer from this the resulting phase position of the load, only part of which is detected accurately in the switch.
So far, there have been no fundamental technical solutions and suggestions for this which enable operation of various topologies of a load resonance converter using one and the same control principle.
In addition, systematic detection of further resonance networks of a resonance converter by means of a control circuit, in addition to the load resonance network, is a task which has not been solved to date, which represents its influence on the regulation and control behavior, for example when a phase difference is to be evaluated as a regulated or controlled variable. In addition, there has so far been no useful technical solution to using a load circuit at the same time for supplying the control circuit when no additional auxiliary tappings are to be provided for supplying the control power and when, at the same time, the load current is not to be corrupted by such an auxiliary current supply with regard to its detection in terms of amplitude and phase position.
Furthermore, there has so far been no technical solution to detecting a variable proportional to the output voltage, in a manner in which it is galvanically separate from the output, such that it is neither electrically connected to a potential of the output voltage nor to a potential of the input voltage at the same time, but may be evaluated at any electrical potential desired, so that the two input electrodes may be guided via a voltage supply circuit, connected upstream from the piezo transformer, for supplying the control circuit, rather than also having to be connected, for example, at the reference potential of the control circuit, which is required for evaluating this auxiliary voltage proportional to an output voltage, the detection being irrespective of whether the output voltage is a pure alternating voltage or a trapezoid or oblong alternating voltage which acts toward the load via a rectifying circuit, and the amplitude of which corresponds to the direct load voltage.
In addition, the detection of the input voltage of power-transmitting converters mostly is implemented by an ohmic resistive divider which requires two highly resistive divider resistors and thus requires an additional terminal at a control IC or an additional terminal at an analog discrete evaluation circuit. Therefore, it is desirable to detect the input voltage indirectly via other signals and variables from the switch unit or the load circuit.
In summary, it may thus be stated that galvanically separated regulation of the output voltage is achieved only at great expense in conventional voltage supply circuits.